Corsair CX Series is a Tier 3 PSU, which mean it only meat minimal requirement for 80+ standards. Also, this series is known for its low quality Capacitor.
I would not use this PSU paired with a R9 280X. I would recommend at least a EVGA 600 B PSU or a 550 W psu of a higher quality such as XFX or Antec HCG series